Legislation prescribes certain standards. In particular legislation may prescribe the time period within which a diagnostic is required to generate a result, which may typically be a YES/NO or NORMAL/FAULT indication.
In order for example to reduce particle emissions of diesel engined vehicles it is common to provide a diesel particle filter (DPF) in the exhaust tailpipe. It may soon also become common to provide a gasoline particle filter (GPF) in the tailpipe of gasoline engine vehicles.
Legislation prescribes that an OBD be provided to confirm that exhaust emission controls are functioning correctly. One such OBD requirement relates to gross failure of the DPF, and is intended to indicate whether the DPF is wholly or partially missing from the exhaust system. Typically this OBD will detect a failure, and illuminate a malfunction illumination lamp (MIL) on the vehicle dashboard, so as to alert the vehicle driver that investigation and/or repair is necessary.
If the OBD is not effective, untreated exhaust emissions may pass to the exhaust tailpipe outlet.
Another area where a rapid result is required from an OBD is in relation to the correct working of safety functions. For example one mode of failure of a rotating component may generally be predicted from detecting an increase in vibration. Just prior to failure, the amplitude of a vibration may increase very rapidly, and a corresponding OBD may be required to deliver a very rapid result in order to detect and identify an imminent failure.
Yet another instance relates to rapid decision making based on data of variable quality, in particular where the accuracy of data recognition is dependent upon, for example, the distance of the measured parameter from a sensor of the measured parameter.
The following is an example of the importance of rapid decision making in relation to an OBD for a vehicle exhaust system.
A characteristic of known OBDs for determining whether a DPF has failed is that they use the measured differential pressure across the DPF as an input. The differential pressure varies in proportion with the magnitude of the gas flow through the DPF and the magnitude of the differential pressure value, the accuracy of the data and the opportunity to identify DPF failures are all at their greatest at high gas flows.
This differential pressure is continuously sampled and converted into an electrical signal. Very typically the electrical signal is noisy, and this noise may show considerable amplitude notwithstanding that the instant condition of a DPF may be substantially unchanging. Electrical noise is due to many factors, such as for example vehicle vibration.
Similarly there may be a great deal of noise on any values calculated by combination of the differential pressure with other appropriate engine or exhaust signals (for example a combined metric comprising differential pressure and volumetric flow rate) due to different response times between the various signals.
A standard technique for reducing such noise in the electrical or calculated signals is to use a substantial low pass filter, either by direct application of an electrical device on the electrical signal line or by application of appropriate digital signal processing within an engine management system; to produce heavily damped signal which will eliminate outliers. The heavily damped differential pressure signal or other calculated signals may then be tested against a threshold to indicate pass or fail.
However such techniques are undiscerning in their application of the filter, giving equal precedence to low accuracy data recorded at low levels of exhaust gas flows and high accuracy data recorded at high gas flows. As such the overall accuracy of the data delivered and judgements made on it will inevitably be compromised.
Another OBD may, in place of the filters, use data from a predetermined time period, and calculate an average from many measured values. These average values may then be combined to produce a signal which is then tested against a threshold to indicate pass or fail. However this method is subject to the same problem exhibited by the low pass filter technique, in that the accuracy of the high quality data recorded at high exhaust gas flows will be compromised by combination with low quality data recorded at low gas flows.
The application of severe entry conditions such that these known OBDs can only operate when presented with high levels of exhaust gas flow, can effectively obviate this issue, ensuring that the OBDs will only operate with high flow/high accuracy data. However for many vehicles types and customer usage profiles these periods of high exhaust gas flow occur infrequently and for very brief periods such as during periods of high vehicle acceleration. As such, the use of such entry conditions may, for many vehicles and customers, prevent these known OBDs from operating with sufficient frequency to achieve legislative operational requirements.
Similar problems can be identified in relation to data of variable accuracy related to other devices or apparatus. For example in relation to vibration of a rotating shaft, the quality of a sensor signal may be affected by noise from other sources of vibration (for example a motor or engine) such that a reliable judgement can be made for low speed data only after a significant quantity of data has been filtered or averaged according to known techniques. On the other hand high speed data may be considered to be a high quality if free from ambient influences, and accordingly a small data set may give a reliable diagnosis.
What is required is a diagnostic able to distinguish and use data of varying levels of accuracy; able to deliver both rapid judgements on data of variable accuracy, and also by ensuring that the accuracy of the best quality data is not compromised, able to subsequently deliver additional better quality judgements as and when sufficient high accuracy data becomes available during a given operational cycle.